One-to-Many Relationship in ERD Design

The Most Common Relationship

The one-to-many (1:N) relationship is the most frequently used relationship type in relational databases. It means that a single record in one table can be associated with multiple records in another table, but each record in the second table is associated with exactly one record in the first.

How It Works

Consider users and posts. One user can write many posts, but each post is written by exactly one user.

CREATE TABLE users (
  id SERIAL PRIMARY KEY,
  username VARCHAR(255) NOT NULL
);

CREATE TABLE posts (
  id SERIAL PRIMARY KEY,
  user_id INT NOT NULL,
  title VARCHAR(255) NOT NULL,
  FOREIGN KEY (user_id) REFERENCES users(id)
);

The foreign key (user_id) lives in the "many" side table (posts). This is the fundamental rule: the foreign key always goes on the "many" side.

Identifying One-to-Many Relationships

Ask yourself: "Can one X have many Ys?" If the answer is yes, and "Can one Y belong to only one X?" is also yes, then it's a one-to-many relationship.

"One" side	"Many" side	Foreign key in
departments	employees	employees.department_id
categories	products	products.category_id
authors	books	books.author_id
customers	orders	orders.customer_id

Crow's Foot Representation

In the ERD editor, a one-to-many relationship displays a single line on the "one" side and a crow's foot (three-pronged fork) on the "many" side. The label "one : many" appears on the connecting line.

Cascading Behavior

When designing one-to-many relationships, consider what happens when the "one" side record is deleted. Common strategies:

• CASCADE: Delete all related records (e.g., deleting a user deletes all their posts)
• SET NULL: Set the foreign key to NULL (e.g., keep the post but remove the author)
• RESTRICT: Prevent deletion if related records exist